The invention relates to a piezoelectric actuator for driving a movable part by means of an elongate piezoelectric resonator, which serves to perform longitudinal vibrations and which is acted upon by elastic means.
The use of piezoelectric drives in small electrical appliances becomes increasingly significant because drives of this type are very robust, compact and flexible in use. The document U.S. Pat. No. 4,019,073 describes such a piezoelectric drive. It relates to a longitudinally vibrating piezoelectric resonator, which drives a shaft. Furthermore, the piezoelectric resonator has its end which faces the shaft secured to a spring, which spring exerts a force perpendicular to the rotor and urges the latter against the force. This has an adverse effect on the efficiency.
It is an object of the invention to provide a piezoelectric actuator having a high efficiency.
According to the invention this object is achieved in that at least one piezoelectric resonator and a spring means, which exerts a force on the piezoelectric resonator at least in the direction of its longitudinal vibrations, are arranged in such a manner that the spring means acts upon the piezoelectric resonator, the resonant frequency of the spring means being substantially lower than the resonant frequency of the piezoelectric resonator.
This solution has the major advantage that the piezoelectric resonator can vibrate in a longitudinal direction of vibration without having to overcome any large resistances. When the resonator expands it exerts a force on the movable part, as a result of which the movable part is set into motion. This force can be exerted only when it experiences a counteracting force. This counteracting force is applied by the elastic means. When the resonator contracts, the counteracting force of the elastic means should be as small as possible. This is possible because the piezoelectric resonator and the elastic means are not secured to one another but merely abut against one another and can also become disengaged from one another as a result of the different resonant frequencies of the piezoelectric resonator and the spring means. The efficiency of a piezoelectric resonator increases according as it can vibrate more freely. In contrast thereto, the piezoelectric resonator in accordance with U.S. Pat. No. 4,019,073 requires a part of the applied electrical energy to overcome frictional resistances and to tension the spring means.
In the embodiment defined in claim 2 the piezoelectric resonator drives a rotor. In order to increase the efficiency even further it is important to find the optimum angle between the piezoelectric resonator and the rotor. This angle depends inter alia on the nature of the material of the resonator and the rotor and the coefficient of friction, which depends thereon. The optimum angle can be calculated and set by means of this coefficient.
The embodiment defined in claim 3 has the advantage that at its end which faces the rotor the piezoelectric resonator cannot move away from the rotor. This ensures in a reliable manner that the resonator cannot leave the rotor.
In the embodiment defined in claim 4 the problem is solved that as a result of the friction between the piezoelectric resonator and the guide elements and the pressure of the resonator on the rotor the angle increases in the course of time because material is eroded as a result of wear. In order to preclude this, the angle is chosen so as to be slightly smaller than calculated, which allows for the increase of the angle during operation.
The embodiment defined in claim 5 improves the power transmission of the piezoelectric resonator. In order to increase the efficiency, it is advantageous to deposit a hard material on the end of the piezoelectric resonator which faces the rotor. This material is wear resistant and at the same time provides the high coefficient of friction required for a correct power transmission between the resonator and the rotor.
The multi-layer resonator defined in claim 6 can operate with substantially lower voltages than a conventional single-layer resonator. This reduces the power consumption.
By means of the embodiment defined in claim 7 it is achieved that less energy is transferred to the guide elements via the contact areas between the resonator and guide elements in a direction normal to the resonator. Therefore, a deliberate acoustic mismatch of the resonator and the guide elements is provided.
According to claim 8 the piezoelectric actuator is used for driving the cutting members in an electric shaver. This provides a substantial reduction of the power consumption and the weight in comparison with shavers having conventional electric motors.
The embodiments defined in claims 9 and 10 concern a shaver having rotary cutting members. The use of the space-saving piezoelectric actuators makes it possible to provide a separate drive for each of the rotors of a shaver having a plurality of rotors. This enables the rotors to be driven fully independently of one another and with different speeds. Conventional electric motors require so much space that particularly in shavers there is room for only one motor. Speed differences between the commonly driven rotors can then be realized only by means of a gear transmission, which renders the construction more intricate and does not variable differences in speed between the rotors.
Hereinafter, embodiments of the invention will be described in more detail by way of example with reference to the drawing.